Is it a rush of blood, rash in the moment thing? Or are you premeditating? I found myself a couple of times this season after I'd got myself in saying "right, if it's in the slot I'm going after this one", and then having to fight myself and say "no, nothing stupid. Wait for it, play it as it comes, nothing stupid". This would happen every couple of overs, and I'd just keep talking myself throught it, "nothing stupid, wait for it, play it on its merits". I found that and having a specific song in my head that I would keep singing helped.
And as MJB83 has mentioned, take your net absolutely seriously and treat it as a game situation (how many of us have blokes at the club who try to club everything in the nets and then get out cheaply on a weekend?). Don't go after balls in the nets that you wouldn't go after in a game, or if you would normally go after try to rein it in. It takes a lot of self discipline, but you will find that after a few weeks it will be working. Be prepared to work on it though, and stick to it.